Abstract

The Upper Cretaceous Cerro Toro Formation in the Silla Syncline, Parque Nacional Torres del Paine, Magallanes Basin, Chile, includes over 1100 m of mainly thin-bedded mud-rich turbidites containing three thick divisions of coarse conglomerate and sandstone. Facies distributions, stacking patterns and lateral relationships indicate that the coarse-grained sandstone and conglomerate units represent the fill of a series of large south to south-east trending deep-water channels or channel complexes. The middle coarse division, informally named the Paine member, represents the fill of at least three discrete channels or channel complexes, termed Paine A, B and C. The uppermost of these, Paine C, represents a channel belt about 3·5 km wide and its fill displays explicit details of channel geometry, channel margins, and the processes of channel development and evolution. Along its northern margin, Paine C consists of stacked, laterally offset channels, each eroded into fine-grained mudstone and thin-bedded sandy turbidites. Along its southern margin, the Paine C complex was bounded by a single, deeply incised but stepped erosional surface. The evolution of the Paine C channel occurred through multiple cycles of activity, each involving: (i) an initial period of channel erosion into underlying fine-grained sediments; (ii) deposition of coarse-grained pebble to cobble conglomerate and sandstone within the channel; and (iii) waning of coarse sediment deposition and accumulation of a widespread sheet of fine-grained, thin-bedded turbidites inside and outside the channel. The thin-bedded turbidites deposited within, and adjacent to, the channel along the northern margin of the Paine C complex do not appear to represent levée deposits but, rather, a separate fine-grained turbidite system that impinged on the Paine C channel from the north. The Cerro Toro channel complex in the Silla Syncline may mark either an early axial zone of the Magallanes Basin or a local slope mini-basin developed behind a zone of slope faulting and folding now present immediately east of the syncline. If the latter, flows moving downslope toward the basin axis further east were diverted to the south by this developing structural high, deposited part of their coarse sediment loads, and exited the mini-basin at a point located near the south-eastern edge of the present Silla Syncline.